FIG. 2.

Principles of competitive ELISA for SPE-A. (A) The serum concentration appropriate for the competitive ELISA was deduced from the linear part of the dose-response curve obtained in the ELISA for SPE-A. (B) Supernatant preparations of the strains were incubated at several concentrations to equilibrium with a fixed serum concentration as determined from panel A. SPE-A-coated ELISA plates were incubated with this equilibrated mixture, and unbound antibody levels were subsequently determined. A 30% inhibition of the OD₄₅₀ was taken as the lower threshold to designate a exotoxin-producing strain.